1. Field of the Invention
This invention relates to a power supply voltage switching circuit, and more particularly, a power supply voltage switching circuit capable of preventing current leakage.
2. Description of the Prior Art
As the integrated circuit technology grows more and more complicate, the requirement of power supply for the integrated circuit also grows. Therefore, power supply for different voltages has been used in many applications, such as non-volatile flash memory. The different voltages may be used for operations in different modes.
FIG. 1 shows a conventional power supply voltage switching circuit 100 according to prior art. The power supply voltage switching circuit 100 includes a level shifting module 110 and a supply switching module 120. The level shifting module 110 includes two level-shifters 112 and 114 for respectively shifting levels of the control signals ENA and ENB to generate shifted signals ENA′ and ENB′. The voltages of the shifted signals ENA′ and ENB′ are determined by the power supply voltage of the level-shifters 112 and 114. The selecting switch module 120 includes two P-type transistors 122 and 124. The transistor 122 and the transistor 124 receive different voltages VPP and VPR. The voltage VPP may be a charge pumped high voltage used for a program operation of the non-volatile flash memory, while the voltage VPR may be a rather low operational voltage used for a read operation of the non-volatile flash memory. Also, the transistors 122 and 124 are controlled by different shifted signals ENA′ and ENB′; therefore, by generating the control signals ENA and ENB and shifting levels of the control signals ENA and ENB properly, the shifted signals ENA′ and ENB′ can be used to turn on or turn off the transistors 122 and 124 and further control the power supply voltage switching circuit 100 to output the desired voltage Vo.
For example, to output the voltage VPR, the control signal ENB may be at a rather low voltage VSS, and the control signal ENA may be at a regular system operational voltage VDD greater than the voltage VSS. In this case, the shifted signal ENB′ would be at the low voltage VSS, and the voltage of the shifted signal ENA′ would be shifted to a voltage near the power supply voltage of the level-shifter 112, that is the voltage VPP. Therefore, the transistor 124 would be turned on and the transistor 122 would be turned off. The turned-on transistor 124 would output the selected voltage VPR. For some systems, the voltage VPP may be 7V, the voltage VDD may be 3.3V, and the voltage VPR may be 1.8V.
However, as the applications vary, the voltages received by the level shifting module 110 and the supply switching module 120 may also vary for operations of different modes. For convenience of switching suitable voltages, the power supply voltage of the level shifting module 110 may be generated by a voltage selecting module. For example, in some situation, the transistor 122 may receive the voltage VDD instead of the voltage VPP. The level shifting module 110 would select the voltage VDD as the power supply voltage for the level shifting module 110. However, the power supply voltage may be a little lower than the voltage VDD due to the limitation of the voltage selecting module. Therefore, according to the aforementioned process, when outputting the voltage VPR, the shifted signal ENA′ would be at a voltage slightly lower than the voltage VDD. In this case, the shifted signal ENA′ may not be able to turn off the transistor 122, and the leakage current may flow from the transistor 122 to the transistor 124. Since the non-volatile flash memory may be operated in a high speed, this kind of leakage current can be hard to predict and can cause lots of power consumption. Therefore, how to reduce current leakage has been a critical issue to be solved.